\begin{tabular}{cccc}
	A    & B   & C     & D                                                 \\
	AAA  & BBB & CCC   & DDD                                               \\
	\multicolumn{2}{c}{first heading} & \multicolumn{2}{c}{second heading} \\
	one  & two & three & four                                              \\
	five &     & six   &                                                   \\
	seven &                                                                \\
\end{tabular}
